Bird song and its production

Almost all birds produce calls of various kinds but one group of passerines, 4000 species called song birds, produce extended and complex songs. A defining characteristic of these songs is that their full expression requires learning by a juvenile bird from mature, singing adults that act as tutors. Song birds are believed to have evolved from one common ancestor, and include finches, warblers and thrushes. The most complex songs are usually produced by males during the breeding season, and the functions of song include establishing territories, attracting potential mates, and maintaining pair bonds (Catchpole & Slater, 1995). Because birds incorporate parts of the songs of other birds into their own songs, local dialects develop in some species (Marler & Tamura, 1964; Baker & Cunningham, 1985).

For a neuroethologist, bird song provides some general lessons about the way a complex type of behaviour is controlled by a central nervous system. Within the brain, a number of discrete areas, or nuclei, have been shown to be involved in song. Each nucleus contains cell bodies and dendrites of a number of types of neuron. Some of the neurons have axons that run in tracts to other nuclei, and other neurons participate in processing information within their nucleus. One group of nuclei is responsible for generating the song in the adult, and another is involved in laying down motor

Box 9.1. Plasticity in the gill-withdrawal reflex of Aplysia

Aplysia withdraws its delicate gill if skin near to the siphon is touched (see section 8.2). Properties of the synapse that links a sensory and motor neuron in the abdominal ganglion (*) change as a result of different kinds of sensory experience, and contribute to simple plastic changes in behaviour. If the synapse is activated repeatedly, the strength of transmission declines and this is one mechanism underlying habituation of the gill-withdrawal reflex. A noxious stimulus applied elsewhere to the body, such as to the tail, sensitises the reflex so that the next siphon stimulus will evoke a larger gill-withdrawal response than previously. Sensitisation is due to the action of facilitating interneurons which release the transmitter serotonin onto the presynaptic terminals of the siphon sensory neurons. This causes presynaptic facilitation, an increase in the amount of neurotransmitter released (Byrne & Kandel, 1996). The circuit can also be conditioned by following a touch to the siphon immediately by a noxious stimulus to the tail. After this pairing, the next touch to the siphon causes a greatly enhanced postsynaptic potential in the gill-withdrawal motor neuron. At least two coincidence-detecting mechanisms operate to enhance synaptic activity. The first is activated when serotonin stimulates the sensory terminal of the sensory neuron while the sensory neuron is electrically excited, and this increases the manufacture of cyclic AMP in the sensory terminal (Hawkins, Kandel & Siegelbaum, 1993). The second is similar to a phenomenon known as post-tetanic potentiation, familiar in some parts of the vertebrate central nervous system. It occurs if the sensory neuron is stimulated at a time when the motor neuron is already excited, and involves a postsynaptic receptor protein called an NMDA receptor (Glanzman, 1995).

Sensory neuron

Facilitatory interneuron

Sensory neuron

J--Siphon

Facilitatory interneuron

Motor neuron

Motor neuron programs for singing. The subject of many studies is the zebra finch (Taeniopygia guttata), an Australian species that breeds readily in captivity and develops to maturity in only 100 days. An individual male produces song that is more easy to characterise than the songs of many other birds because it is relatively stereotyped.

A lot of information about a song can be expressed in a sonograph, in which records of sounds are broken down to show the relative contributions of different frequencies. Sonographs are particularly useful for comparing the songs of different individuals, or of one bird at different stages in its development. The sonograph in Fig. 9.6a illustrates different levels of organisation within the song of a zebra finch. Series of notes are linked together into discrete syllables, and a series of syllables is linked together in a unit called a motif. When birds are interrupted during singing, they always finish a syllable (Cynx, 1990), which means that a syllable is a basic unit in the organisation of song. In zebra finches, the motif that a particular male sings is fixed in form, although the number of motifs in a bout of singing varies from song to song, as do the brief introductory notes that precede the song and separate successive motifs. Each note of the zebra finch song is composed of sounds of many different frequencies. Notes of most other species contain more restricted tones and, as a result, have a more musical quality to a human listener.

The organ responsible for producing sounds during song is the syrinx, located where the trachea joins the bronchi of the two lungs (Fig. 9.6b). Four to six muscles on either side are attached to the syrinx, and sound is produced when air is expelled through it. The exact way in which sound is produced has not been conclusively demonstrated, but it may involve the openings of the bronchi into the trachea forming whistles, or the two medial tympaniform membranes vibrating like drum membranes. Some syrinx muscles determine the tone of sound which is produced and others control the timing of sounds by opening and closing the bronchi. Respiratory muscles generate the force for expelling air through the syrinx, and so control the volume of sound. The lungs of birds are rigid and air moves through them in one direction by the action of large air sacs that act as bellows. Each syllable of a song is produced by contraction of muscles that expel air from the interclavicular air sac. Electromyograms and pressure measurements have shown that, in canaries, each syllable is co-ordinated with a cycle of inspiration and expiration, even at rates in excess of 20/s. It is

Figure 9.6 Bird song and its production. (a) A sonograph of the song of a zebra finch (Taeniopygia guttata). Shown here are a few introductory notes, followed by three repetitions of the same motif. This motif contained six syllables, each one of which was a particular sequence of notes. (b) The main structures associated with the syrinx of a song bird. (a sonograph kindly supplied by Dr D. Margoliash; b from Suthers, 1990; reprinted with permission from Nature; copyright © 1990 Macmillan Magazines Ltd.)

Figure 9.6 Bird song and its production. (a) A sonograph of the song of a zebra finch (Taeniopygia guttata). Shown here are a few introductory notes, followed by three repetitions of the same motif. This motif contained six syllables, each one of which was a particular sequence of notes. (b) The main structures associated with the syrinx of a song bird. (a sonograph kindly supplied by Dr D. Margoliash; b from Suthers, 1990; reprinted with permission from Nature; copyright © 1990 Macmillan Magazines Ltd.)

usual for the muscles on the left and right of the syrinx to act independently of each other, and zebra finches sing mostly by using muscles on the right.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Post a comment